For implantation of prosthetic stems, such as hip stems, accurate preparation of the bone or intramedullary canal is extremely important in order to guarantee good contact between the prosthesis stem and the bone. The underlying concept behind precise preparation is that a precise bone envelope reduces the gaps between the implant (i.e. prosthesis or prosthetic component) and the bone, thereby improving the initial and long-term bone ingrowth/fixation. The bone canal is presently prepared for implantation of a prosthetic stem by drilling and reaming a resected end of a bone, such as a femur, and then preparing an area adjacent the drilled hole to provide a seat for the prosthetic stem or a proximal sleeve coupled to the stem of a modular prosthetic system.
Modular prosthetic systems using proximal sleeves, stems, necks and heads, such as the S-ROM Modular Hip System, available from DePuy Orthopaedics, Warsaw, Ind., put more control in the hands of the surgeon, providing solutions for a variety of surgical scenarios, from primary total hip arthroplasty (THA) to the complex revision or DDH challenges. Such system provides such versatility because the provided plurality of stems, sleeves, necks and heads which can be assembled in a large number of configurations.
Preparation of the area adjacent the drilled hole may be accomplished by broaching or by milling. Broaches or rasps, when used for bone preparation, have limitations. One such limitation is the risk of fracture during broaching. Since broaching is done by pounding the broach into the bone, the bone tends to fracture. Additionally, both broaches and rasps suffer from a tendency to be deflected by harder sections of bone so that they do not create as precise a triangular cavity as can be created by a miller system. In a study that compared an intimate fill with robotically machined femoral, Paul et al., found that broaching tore the trabecular bone, whereas femoral canal preparation with reamers was consistently more accurate. Paul, H. A., et al. “Development of a Surgical Robot for Cementless Total Hip Arthroplasty.” Clinical Orthopedics and Related Research 285 December 1992: 57-66.
Thus, milling is currently the preferred method of bone preparation in many orthopaedic applications because it is an extremely precise method of bone preparation. A limitation of milling systems today is that they are typically formed so that the drive shaft extends at an angle relative to the remainder of the frame from the end of the miller cutter machining the bone. A fairly large incision must be made to accommodate such milling assemblies. A typical incision for preparing a femur for a total prosthetic hip replacement using a standard triangle miller system is eight to ten inches long. It is not uncommon for incisions as large as 12 inches to be used in a total hip replacement procedure.
A standard triangle miller system typically includes a miller shell, a miller frame and a miller cutter having an end formed for coupling to a drill. A typical miller frame and miller cutter can be seen in U.S. Pat. No. 5,540,694 issued to DeCarlo, Jr. et al. on Jul. 30, 1996. This miller frame allows for precise machining of the triangular canal by a miller cutter held at an angle with respect to the shaft of the frame. The triangular canal facilitates an accurate fit of a proximal sleeve that distributes the load experienced by the prosthesis evenly and provides rotational stability. However, to accommodate this miller, it is necessary to make a fairly large incision which may be undesirable for cosmetic or other reasons.
The large incision is required because the miller cutter includes a fixed input shaft for connecting to and/or receiving motive (i.e. rotary) power from a drill or similar instrument. As such, the prior reamer is able to accept rotary input power with respect to only one direction. Typically, this direction is at 0° (i.e. “straight on”) with respect to the reamer which is approximately thirty two degrees with respect to the shaft of the miller frame. Therefore, not only is the input power direction restricted, but this, in turn, restricts the angle at which the reamer may be used on a patient. Since the input shaft and the drill coupled thereto extend laterally beyond the edge of the miller frame an incision substantially larger than the width of the frame must be made to accommodate the reamer, frame and drill during surgery. The incision must be large enough to accommodate the reamer, frame, input shaft and drill without the input shaft engaging soft tissue.
Recently, there have been some millers developed for minimally invasive surgery, however, they still require an incision of about three inches. However, as other instruments only require about one and a half to two inches, it is not ideal to use a miller that requires a larger incision.
In view of the above, it would be desirable to have a bone miller or guided reamer that could fit into a smaller incision during a surgical process.